1. Field of the Invention
The present invention relates to a honeycomb heater which can be used as a preheater for controlling automobile exhaust emissions or as an industrial heater. The honeycomb heater may be used as a component in a catalytic converter for controlling automobile exhaust emission.
2. Description of the Related Art
Conventionally, porous ceramic honeycomb structures have been employed as carriers for a catalyst composition for removing, for example, nitrogen oxides, carbon monoxide, and hydrocarbons in the exhaust gas of internal combustion engines of automobiles. Recently, a honeycomb structure composed of a metallic material has drawn attention since the metallic honeycomb structure has high mechanical strength and heat resistance. The metallic honeycomb structure may be made by firing a green compact including metal powders.
As restriction of exhaust emission controls has been intensified, there has been a demand for heaters adapted to decrease pollutants during the initial running of the engine when the engine is not warmed up. The removal of hydrocarbons, carbon monoxide, nitrogen oxides while an exhaust gas is cold, is particularly important.
When the temperature of an exhaust gas is low during the initial running of the engine, the catalyst composition has a low activity for oxidizing hydrocarbons in the exhaust gas. The conversion rate of hydrocarbons by a catalyst composition is, for example, 5% or less when the exhaust gas is about 200.degree. C., 50% when the exhaust gas is about 300.degree. C., and at least 95% when the exhaust gas is about 400.degree. C. Moreover, during the initial running of the engine, the engine emits a large amount of hydrocarbons as compared with hydrocarbons emitted during the continuous operation. Hence, it is preferable to heat the exhaust gas during the initial running of the engine to a light-off temperature above which the catalyst composition is sufficiently activated to remove pollutants in the exhaust gas.
An apparatus for controlling an exhaust gas of the engine in the exhaust system of automobile generally comprises a preheater and a main catalyst element (e.g. a three-way catalyst element), and the preheater is provided upstream of the main catalyst element.
Japanese Patent Application Laid-Open No. 295184/1991, which corresponds to U.S. Pat. No. 5,063,029, discloses a resistance adjusting type heater and a catalytic converter. This resistance adjusting type heater comprises an electrically conductive honeycomb structure, at least two electrodes in electrical contact with the honeycomb structure, and means for adjusting the electrical resistance of the honeycomb structure. The means may be a slit extending through partition walls defining passages of the honeycomb structure. A catalyst composition may be loaded onto surfaces of partition walls.
This resistance adjusting type heater can control its heat generation by regulating the electric resistance of the honeycomb structure. Preferably the heater reaches high temperatures in a short time with a high temperature elevation rate when it is subjected to post-heating (i.e. it is heated simultaneously with the start of the engine).
However, it still takes a somewhat long time to heat the entire honeycomb structure to the so-called light-off temperature of the catalyst composition. In order to reduce the heating time, it was attempted to reduce the volume of the heater so as to reduce a heat capacity of the honeycomb structure; however, it was difficult to treat hydrocarbons in an exhaust gas during the initial running of the engine, for example, during a period of up to about one minute from the time the engine is started.